The biological functioning of some proteins is critically dependent on their state self-association or polymerization. Knowledge of the stoichiometry and thermodynamic properties of the association is quite useful in forming hypotheses on the mechanism. We are engaged in such measurements on the polymerization of protein from the tobacco mosaic virus, a process thought to resemble at least in part the assembly of the virus in the plant cell. Specifically, our approach depends heavily on equilibrium centrifugation, and its interpretation in terms of formation of structural intermediates. Not only is this approach useful in itself, but is likely that it can be directed toward other systems. The reasons for this supplemental request arise because the research in the original proposal has turned out to be rather successful. Several avenues of investigation have opened up, avenues that were not apparent at the time the original proposal was made. The results obtained already and the new experiments are described in the research plan of this proposal. It is now necessary to provide for increasing the speed and efficiency of doing experiments.